1. Field of the Invention
The present invention relates to a motor-driven focusing apparatus which has a manual operating switch to finely adjust the focal point of a telescopic optical system of a sighting telescope incorporated in a surveying instrument such as an automatic level, a transit, a theodolite, etc.
2. Description of the Related Art
A conventional surveying instrument such as an automatic level (auto-level), a transit, a theodolite, etc., is generally provided with a sighting telescope (collimating telescope), a level and a measuring device which measures a rotational angle, a descending angle, and an ascending angle, etc. When this type of surveying instrument is used, the sighting telescope is positioned horizontally and subsequently the horizontal and vertical adjustments for the sighting telescope are performed. Thereafter, the sighting telescope is aimed at a reference object or a reference point so that the reference object is sighted by a surveyor.
For instance, the optical system of the sighting telescope of an automatic level includes an objective lens group, a focusing lens group and an eyepiece, arranged in that order from the object side. The position of the focusing lens group is adjusted depending on the object distance, so as to form a sharp object image on the reticle provided on the focal plane. The object image formed on the reticle can be viewed through the eyepiece.
The amount of movement of the focal point per unit amount of movement of the focusing lens group varies depending upon the axial position of the focusing lens group on the optical axis thereof. FIG. 7 shows an example of the relationship between the axial position of the focusing lens group (the object distance) and the FL factor. The FL factor represents the amount of movement of the focal point when the focusing lens group moves by a unit amount of movement; namely, the FL factor represents the value of the amount of movement of the focusing lens group divided by the amount of movement of the focal point. The vertical axis of the graph shown in FIG. 7 represents the FL factor while the horizontal axis of the graph represents the axial position of the focusing lens group wherein the image of an object positioned at the indicated object distance (meters) is focused on the reticle. It can be understood from the graph shown in FIG. 7 that the amount of movement of the focal point is not always proportional to the amount of movement of the focusing lens group.
Due to this fact, in the case of a sighting telescope provided with a motor-driven focusing apparatus (auxiliary focusing apparatus) used for finely adjusting the focal point to form a sharp object image on the reticle by operating a manual operating switch (e.g., a controlling knob or lever) of the motor-driven focusing apparatus after the object is brought into focus by the autofocusing mechanism of the sighting telescope, it is difficult to finely adjust the focal point to form a sharp object image on the reticle with the motor-driven focusing apparatus since the amount of movement of the focal point per unit amount of movement of the focusing lens group varies depending on the axial position of the focusing lens group on the optical axis thereof.